The object of this research is to study the factors which affect favorably or adversely differentiation and anaplasia in tumors of the central and peripheral nervous system, and to study problems of host- tumor relationship, including the stroma-inducing properties of neuroectodermal tumors. Suitable in vitro systems for the investigation of a number of human and experimental neoplasms of the nervous system selected for their applicability to the study of these problems have been established. Experimental nervous system tumors are obtained in rats by transplacental induction, using a single intravenous injection of ethyl nitrosourea in the pregnant mother in the last week of pregnancy. In the case of human tumors, emphasis in these studies is placed on those neoplasms in which anaplasia and dedifferentiation are the most evident, namely medulloblastomas and glioblastomas. The phenomena to be studied will include cell differentiation, maturation, regressive anaplasia and stroma-induction. The approach includes light and electron microscopy, observations in tissue culture and organ culture systems, manipulation of the environment of cell lines to study differentiation, and the investigation of stroma-induction in diffusion chambers. An attempt will be made to correlate morphological observations with biochemical analyses of these systems, including the use of labelled precursors for the study of protein, RNA and DNA synthesis.